Progesterone plays an important role in the regulation of your menstrual cycle. This hormone also helps maintain a uterine lining suitable for the implantation of an embryo if you get pregnant. A low progesterone level can affect your menstrual cycle, cause symptoms in perimenopause or threaten a pregnancy. However, you might not notice any symptoms from a low progesterone level. Your doctor can check the levels of progesterone in your blood or by studying a piece of tissue from your uterine lining.
Normal Progesterone Levels
Progesterone levels fluctuate during the menstrual cycle. Your progesterone level is normally low -- less than 1.5 nanograms per milliliter, or ng/mL -- until you ovulate, or release a mature egg from the ovary. The corpus luteum, which is the leftover shell of the follicle that held the egg, produces progesterone after ovulation. Progesterone helps stimulate thickening of the uterine lining, preparing it for an embryo to implant. Roughly 7 days after ovulation, progesterone levels should rise to 10 ng/mL or greater. A level below this is considered low. During pregnancy, progesterone levels remain at post-ovulation levels or higher.
Low Progesterone and Your Menstrual Cycle
If the corpus luteum does not produce an adequate amount of progesterone, your uterine lining will not thicken as it should. Normally, progesterone maintains the uterine lining for approximately 14 days. After this, the corpus luteum begins to deteriorate, progesterone levels fall and the uterine lining starts to slough off. This is what happens during a normal period. If your progesterone level is low, the lining might begin to shed early and you might get your period earlier than expected. Low progesterone can also cause spotting between ovulation and menstruation. A review of studies published in "BMJ" in October 2001 reported that low progesterone levels do not cause premenstrual symptoms, and taking progesterone does not improve PMS. However, a March 2012 review of available studies from the "Cochrane Database of Systematic Reviews" concluded that it remains unclear whether progesterone is beneficial for the treatment of PMS. Additional research is needed to clarify the role of progesterone as a possible cause or treatment for PMS.
Low Progesterone During Pregnancy
At around the eighth week of pregnancy, the placenta has developed sufficiently to supply the progesterone your body needs to maintain the uterine lining. Until that point, the corpus luteum supplies the necessary progesterone. If you have low progesterone after ovulation, the uterine lining might not develop well enough to maintain a pregnancy. You could experience early recurrent pregnancy loss, possibly even before you are aware that you are pregnant. If you are undergoing fertility treatment, your doctor might check your progesterone levels after ovulation and measure the thickness of your uterine lining via ultrasound. In 2011, the U.S. Food and Drug Administration approved the use of progesterone in pregnancy to prevent preterm birth in women with a history of pregnancy loss. The American College of Obstetrics and Gynecology supports this use.
Low Progesterone and Menopause
During perimenopause -- the time when menstrual periods become irregular before they cease completely -- progesterone production drops. Spotting can occur when estrogen levels remain high but progesterone levels fall. Low progesterone levels might also contribute to bone loss in perimenopausal women, an August 2010 article in the "Journal of Osteoporosis" reports. Although bone loss typically causes no symptoms, it can be detected with special tests and may increase your risk for fractures.